Process and apparatus for melting and solidifying continuously refractory materials



Pi- 1967 R. P. GOTON ETAL 93.

' PROCESS AND APPARATUS FOR MELTING AND SOLIDIFYING CONTINUOUSLY REFRACTORY MATERIALS Filed Oct. 21, 1965 2 Sheets-Sheet 1 Se t. 26, 1967 R. P. GOTON ETAL 3,343,593

PROCESS AND APPARATUS FOR MELTING AND SOLIDIFYING CONTINUOUSLY REFRAQ'I'ORY. MATERIALS Filed Oct. 21, 1965 2 Sheets-Sheet 2 W 9 Mm.

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United States Patent 3,343,593 PROCESS AND APPARATUS FOR MELTING AND SOLIDHFYING CONTHNUOUSLY REFRACTORY MATERIALS Roland Pierre Goton, Avignon, and Joseph Recasens, Sorgues, France, assignors to LElectro-Refractalre, Paris, France, a company of France Filed Get. 21, 1965, Ser.No. 499,444 Claims priority, application France, Dec. 29, 1964, 232, Patent 1,430,962; July 9, 1965, 24,197, Patent 88,438

3 Claims. (Cl. 164-273) Since their introduction electrically cast refractories have been intermittently produced by the melting of the refractory material or mixtures of refractory material in an electric furnace, followed by the casting of the molten refractory product into moulds in which it solidifies into shaped pieces. Each mould takes a volume of liquid which is inadequate for the volume of the final solid product, which contracts as it solidifies, the contraction causing shrinkage cavities in the resultant solid product.

To obviate this disadvantage, during casting one or more casting heads filled with liquid are disposed above the mould. It has also been suggested to keep the material in the feeding head molten, for instance by means of an auxiliary arc, to fill the shrinkage cavities as they occur during the solidification of the refractory block.

However, it has never proved possible to completely avoid shrinkage cavities in electrically cast refractories. To obtain absolutely solid blocks, those portions thereof which contain residual shrinkage cavities have to be sawn off, and this means a very high loss of cast and moulded material.

In the metallurgical industry, which is faced with a fairly similar problem, it has been suggested to use continuous casting methods in which molten metal is regularly and continuously cast from a casting ladle or a melting furnace via. a flow regulator into a vigorously cooled open bottomed receptacle from which there emerges continuously and at a suitably controlled speed a billet whose still liquid centre is finally solidified by a secondary cooling. The billet is subsequently cut up by chisels or torches.

It would be ditficult to use a method of this kind for refractory products, since the constituents of which they are composed require very high temperaturesin the ,order of 2,000 C. and more--to melt them and keep them in the liquid state. They very quickly acquire a pasty constituency as soon as they have left the heating source.

The method according to the invention, which solves the problem of continuously melting and solidifying refractory materials, is characterised in that the initially granular pulverulent oxide or mixture of oxides is continuously passed first through a melting zone which comprises a heating source and in which the oxide is heated by high-frequency electric induction, and then through a solidifying zone immediately downstream of the melting zone, the speed of the movement of the material at the outlet from the melting zone being such that taking into account the heat input to the melting zone and the heat loss immediately after leaving the melting zone, a solidification front is produced which is fixed in space and which is in permanent contact with the liquid material in the melting zone.

The method according to the invention obviates all transference of a volume of liquid from one receptacle to another and enables the shrinkage cavities to be con- "ice tinuously filled with the liquid material in contact with them, the result being a solid refractory billet which can then be cut up into blocks, for instance by diamond cutting Wheels. The method according to the invention also obviates moulding, which is a very expensive operation in the case of electrically cast refractories having high melting pointsi.e., 2,000 C. and above.

In a preferred embodiment of the invention, the material in the melting zone is heated by high-frequency electric induction. Preferably, the wall of the melting zone, which keeps the material liquid, is formed by a metal surface cooled by the circulation of a suitable fluid, and which forms at the same time the high-frequency inductor. The wall therefore simultaneously acts as an inductor, a melting crucible and a moulding element for the continuously emerging billet.

The method according to the invention applies to refractory products of all kinds, whether refractories based on oxides, such as alumina, zirconium oxide, magnesia thoria, etc. or refractories such as carbides, nitrides, borides, etc. However, since the latter react strongly when molten with atmospheric oxygen, the material being melted has to be kept away from the atmosphere. The method according to the invention can therefore be performed, during the whole period of the operating run, by keeping the molten refractory material in the high-frequency melting furnace confined between the solidified refractory material at its base and the freshly supplied pulverulent refractory material, which acts as an insulating crown, at its top, so that atmospheric action is reduced to a negligible amount.

By way of further precaution, an inert atmosphere may be formed, for instance by a rare gas such as argon or nitrogen, maintained or circulated in a small enclosure disposed around the high-frequency melting furnace. An enclosure of this kind requires two sealed inlets for electric current and cooling fluid, and a passage having a sliding gland to enable the rod supporting the moving base of the melting furnace to move freely without the risk of undesirable entry of air.

Similarly, there can be disposed around the high-frequency melting furnace an enclosure of the kind hereinbefore described, and adapted to contain an inert gas at a pressure adequate to limit the losses by evaporation or sublimation of refractory compounds such as nitrides, .which have a substantial vapour pressure at elevated temperatures, and which decompose near their melting points.

Clearly, control of the frequency of the electric induction current and the determination of the dimensions of the inductive coil depend on the electric and thermal conductivity of the refractory compound to be treated. It is therefore impossible to give any detailed indications, since each case mustbe judged on its own merits. However, any engineer in the art will be able to select the correct operational conditions for each particular refractory compound to be treated.

The invention also covers the apparatus for the performance of the method and the product resulting therefrom. The main portion of the apparatus is that which produces inside the charge induced high-frequency currents which evolve the heat necessary for melting. Advantageously, the main portion of the apparatus is constructed in the same way as the high-frequency furnace described in French patent application No. 234 (filed on December 29, 1964 in the name of lElectro-Refractaire), except that according to the present invention the crucible of the furnace whose wall is cooled by the circulationof fluid has itself an open end, the bottom being formed during the solidification process by the solidification front of the material which is continuously emerging from the base of the cylinder crucible.

A non-limitative exemplary embodiment of the invention will now be described with reference to the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic vertical section through the axis of an apparatus for the continuous melting and solidification of refractory compounds such as refractory oxides which do not react with the atmosphere at an elevated temperature,

FIGURE 2 is a cross-section, taken along the line 11-11 in FIGURE 1, and

FIGURE 3 is a diagrammatic view in vertical section through the axis of one embodiment of the apparatus according to the invention for continuously melting and solidifying refractory compounds which react with the atmosphere at an elevated temperature.

The apparatus illustrated in FIGURES 1 and 2 comprises a melting zone A followed by a solidifying zone B and an annealing zone C. The melting zone A comprises a substantially cylindrical double wall 1 which is made of a highly conductive metal, such as copper, and which can have any desired cross-section corresponding with the cross-section of the solidified billet of refractory material to be produced. The double wall 1 is formed with a gap which is closed by a refractory insulating joint 2, and has a configuration such that the metal of the double wall 1 forms a single-turn inductor coil whose two ends are connected to the poles of a high-frequency electric source (see FIG. 2). An inlet 3 and an outlet 4 at the bottom and top respectively of the double wall 1 allow the circulation of cooling water. The melting furnace thus formed is open at the top, into which the charge material is continuously poured from a spout 5.

The moving base of the furnace is formed in the first place, for instance, by a piston 7 which is made of refractory material or has an internal circulation of fluid, and which gradually moves downwards at a rate adjusted so that the solidification front 8 is fixed in space and always stays in contact with the liquid 9 in the melting zone. During operation, therefore, the base of the furnace is formed by the material 8 which is subjected to a degree of cooling adapted to provide the required crystallisation conditions, and solidifies as it moves away from the melting zone. The result is a billet which is completely solid except at its top end on the conclusion of operations. The top end can be sawn off and the billet can be cut up into blocks which are completely sound, i.e., they have no shrinkage cavities.

Since the conductivity of refractory oxides is relatively low when they are cold, at the start of the melting opera tion pre-heating of the pulverulent charge filling the furnace will be required, which charge is retained therein by the piston 7, which is then in its top position and forms the bottom of the furnace. The pre-heating can be provided by any known means, more particularly by an auxiliary electric are or the flame of a torch operating at the top of the furnace.

Preferably, the method can be used which is described in copending US. application Ser. No. 498,134 filed Oct. 19, 1965 in the name of Joseph Recasens, and which comprises associating with the charge which fills the furnace at the start of operations, an auxiliary charge of an additional element which fulfills the two following conditions, namely that when it is cold it has adequate conductivity to be permeated by induced current; and that when it is thus heated in air it can evolve an exothermic reaction which heats the main charge material to a temperature at which the main charge becomes conductive enough to be permeated by induced currents.

The additional initiating element can be deposited on the charge, or preferably deposited in a small hole made in the centre of the charge. The additional element can be, for instance, granular or pulverulent aluminium, zirconium, silicon, magnesium, etc. all of which elements when in contact with atmospheric oxygen become oxidised when heated by induced currents, and thus give off a very great quantity of heat. Of course, it is an advantage to select an element whose oxide is already present in the charge for melting, so as to prevent the melt from being contaminated by an undesirable substance.

FIGURE 3 illustrates an apparatus which is similar to that illustrated in FIGURE 1 and which is adapted for the continuous melting and solidification of refractory compounds which react with the atmosphere at elevated temperature.

To this end, the apparatus illustrated in FIGURES 1 and 2 is disposed in an enclosure 10 comprising seal-tight passages 11, 12 for supplying electric current and cooling fluid, and an aperture 13 having a sliding gland 14 to allow a rod 15 supporting the moving base of the melting furnace, to move freely without the risk of undesirable entry of air. A feeding funnel 16 at the top of the enclosure 12 above the melting tank supplies the same with pulverulent refractory charge material. An inlet 17 and an outlet 18 for an inert gas are provided at the bottom and top respectively of the enclosure 10.

The only difference between the operation of the apparatus illustrated in FIGURE 3 and the apparatus illustrated in FIGURES 1 and 2, is that in the apparatus illustrated in FIGURE 3 a certain quantity 19 of pulverulent refractory charge material is kept by way of an insulating crown on top of the refractory material being melted, and an inert gas being circulated inside the enclosure 10.

Since some carbides, such as silicon carbide, have a relatively high conductivity, it may be necessary to cover the inside wall of the inductive coil with a thin layer of insulating substance sprayed, for instance, by a plasma torch. In the case of silicon carbide, the thin layer can be formed for instance by silica.

The method and apparatus according to the invention enable cast and moulded refractory products to be manufactured which have a very satisfactory quality, and have no trace of oxidation.

Clearly, the embodiments disclosed hereinbefore are merely given by way of example and could be modified, inter alia by the substitution of technical equivalents, without exceeding the scope of the invention.

We claim:

1. In an apparatus for continuously melting and solidifying refractory products comprising at least one refractory oxide, said apparatus comprising a highfrequency inductive melting zone of substantially cylindrical shape whose cross-section is the same as that of the solidified billet of refractory material to be produced, and a moving base for removing the solidified material, the improvement which comprises an electrically conductive metal wall defining said cylindrical melting zone and having a longitudinal gap, a refractory insulating joint obturating said gap, means for externally cooling said Wall by fluid circulation and means for supplying high-frequency current to said wall whereby said wall forms a single-turn inductor coil in addition to being a crucible and a mold for the melted material.

2. In an apparatus for continuously melting and solidifying refractory products which react with the air at elevated temperatures, said apparatus comprising a high-frequency inductive melting zone of substantially cylindrical shape whose cross-section is the same as that of the solidified billet of refractory material to be produced, and a moving base for removing the solidified material, said melting Zone and said moving base being disposed inside an enclosure wherein an inert atmosphere is provided, the improvement which comprises an electrically conductive metal wall defining said cylindrical melting zone and having a longitudinal gap, a refractory insulating joint obturating said gap, means for externally cooling said wall by fluid circulation and means for supplying high-frequency current to said Wall whereby said Wall forms a single-turn inductor coil in addition to being a crucible and a mold for the melted material.

3. In an apparatus for continuously melting and solidifying refractory products which react with the air at elevated temperatures, said apparatus comprising a high-frequency inductive melting zone of substantially cylindrical shape whose cross-section is the same as that of the solidified billet of refractory material to be produced, and a moving base for removing the solidified material, said melting zone being disposed inside an enclosure wherein an inert atmosphere is provided, the improvement which comprises an electrically conductive metal wall defining said cylindrical melting zone and having a longitudinal gap, a refractory insulating joint obturating said gap, means for externally cooling said wall by fluid circulation and means for supplying high-frequency current to said wall whereby said wall forms a single-tum inductor coil in addition to being a crucible and a mold for the melted material.

References Cited UNITED STATES PATENTS Prince 264-27 Thompson 264-27 Eldred 164-89 Meier 164-51 Brennan 164-89 Herres et al. 13-9 Brennan 164-65 Gruber et a1 164-254 X Schaaber 164-251 Hughes 164-61 Brennan 164-64 Schaaber 164-147 Luteijn 164-82 Bussard et al. 266-33 J. SPENCER OVERHOLSER, Primary Examiner.

0 V. K. RISING, Assistant Examiner. 

1. IN AN APPARATUS FOR CONTINUOUSLY MELTING AND SOLIDIFYING REFRACTORY PRODUCTS COMPRISING AT LEAST ONE REFRACTORY OXIDE, SAID APPARATUS COMPRISING A HIGHFREQUENCY INDUCTIVE MELTING ZONE OF SUBSTANTIALLY CYLINDRICAL SHAPE WHOSE CROSS-SECTION IS THE SAME AS THAT OF THE SOLIDIFIED BILLET OF REFRACTORY MATERIAL TO BE PRODUCED, AND A MOVING BASE FOR REMOVING THE SOLIDIFIED MATERIAL, THE IMPROVEMENT WHICH COMPRISES AN ELECTRICALLY CONDUCTIVE METAL WALL DEFINING SAID CYLINDRICAL MELTING ZONE 